Values from CT images of the head are used in different ways during treatment planning for all tFUS procedures. One important metric is the Skull Density Ratio (SDR), an estimate of the transparency of the skull to ultrasound. The SDR is not always predictive of the energy needed to generate a focal spot transcranially, but a lower SDR is generally interpreted to mean lower acoustic transmission through the skull[1]}, [2]}. Although the precise method for computing SDR on a clinical system is proprietary, the metric is derived from the ratio of the Hounsfield Units (HU) of trabecular to cortical bone along the line from a transducer element to the focus[2]}, and an open source software, Kranion, is available that is capable of generating SDR metrics highly correlated to those found in clinical procedures[4]}. Along with SDR, Kranion can report a Skull Thickness (ST) measurement between bone layers and Number of Active Elements (NAE), or an element's ray less than \(<\)  20 degrees incident to the skull. Detailed spatial maps have been created from CT images to map acoustic properties and model the propagation of sound through the skull [5]}, [6]}, [7]}. Using modeling tools like the acoustic toolbox, k-Wave[8]}, simulations are used to observe ultrasound waves interacting with subject-specific heterogeneous skulls, quantifying the focal shift, focus size, and energy loss caused by the aberrating skull.
